X-ray tube power supply having a condenser and/or rectifier in the primary circuit



H. JACOBS Dec. 2 7, 1966 X-RAY TUBE POWER SUPPLY HAVING A CONDENSERAND/OR RECTIFIER IN THE PRIMARY CIRCUIT 2 Sheets-Sheet 1 Filed Nov. 27, 1964 INVENTOR HANS JACOBS Dec. 27, 1966 H. JACOBS 3,294,973

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' g INVENTOR E :1 HANS JACOBS United States Patent 3,294,973 X-RAY TUBE PGWER SUPPLY HAVING A (IGN- DENSER AND/OR RECTIFIER IN THE PRIMARY CERCUET Hans Jacobs, Gevern Block 32, Hamburg- Fuhlshnttei, Germany Fiied Nov. 27, 1964, Ser. No. 414,075 Claims priority, application Germany, Dec. 14, 1963, 3 24,934 9 Claims. (Cl. 250-98) The invention relates to X-ray apparatus of the type in which an X-ray tube is connected directly to a highvoltage transformer having primary windings to which electrical impulses are supplied. The invention further relates to methods of generation of said impulses for supply to the X-ray tube.

From the infancy of X-ray techniques it has been known to operate X-ray apparatus by means of impulses. In the earlier stages of development a spark coil was used for producing the impulses, and in subsequent stages of development a condenser was utilized which was charged and discharged through the primary winding of the spark coil. After the introduction of A.C. current the spark coil was replaced by the more compact transformer, and up to the present, simple apparatus is known with a transformer for direct feeding of the X-ray tube. This system, however, suffers from the following serious deficiencies.

(a) During the periods of high-voltage varying according to a sine curve, usable X-rays are produced only in a short period at the positive maximum of the voltage, whereas the X-ray tube is being heated during the entire positive half-wave.

(b) To prevent loss of the required rectifier effect to the X-ray tube where direct coupling to a transformer is used, the specific load on the focal spot of the tube must be low, which means that the focal spot must be relatively large and consequently will give optically poor pictures.

(c) During the entire negative half-period of the voltage, the apparatus is exposed to electrical influences which do not serve any useful purpose.

It is also possible to drive a transformer, like the spark coil with closed core, by means of the charging and discharging impulses of a condenser in the same manner as explained above, but in such case the effective part of the high-voltage will be still more short lived than when using sinusoidal A.C. current, while the other above-noted deficiencies will not be remedied.

It is an object of the invention to provide X-ray apparatus of the aforesaid type in which the X-ray tube is used more economically than in the apparatus of the known art.

It is a further object of the invention to provide X-ray apparatus of the aforesaid type which is economical and which does not rely on the rectifier effect of the tube.

It is still a further object of the invention to provide X-ray apparatus in which the useless electrical influences are reduced to a minimum.

These and further objects are achieved, at least partly, by apparatus in which the impulses are produced by the charging and discharging of a condenser which is connected to the primary winding of the transformer such that the charging and discharging impulses will produce fluxes in the same direction in the transformer, whereby the voltage impulses supplied to the X-ray tube from the secondary winding of the transformer all have the same direction whether originating from the charging or from the discharging of the condenser. It is noted that the charging and discharging impulses are followed by a series of negative and positive voltage fluctuations because the condenser, together with the primary winding Patented Dec. 27, 1966 of the transformer, forms a resonant circuit, but this is of no significance since this circuit is so attenuated that the fluctuations will be quite small and negligible.

When the X-ray apparatus is of the type in which the charging and discharging of the condenser is controlled by means of a change-over switch, the desired effect may preferably be achieved by providing the transformer with two primary windings connected to the voltage source and to the condenser via the change-over switch such that the charging current passes through one primary winding and the discharging current passes through the other of the primary windings. In this embodiment the apparatus can be fed directly from a DC. voltage source, and when A.C. voltage is to be used the desired effect can be achieved in an equally simple manner by providing each of the primary windings with a center tap connected to each one of the terminals of the A.C. voltage source and by providing the apparatus with two change-over switches connected and synchronized with the A.C. voltage so that they connect the condenser to the respective windings each during a half-period of the A.C. voltage.

In another embodiment of A.C. operated X-ray apparatus according to the invention in which the charging and discharging of the condenser is controlled by a change-over switch, the transformer has only one primary winding and no tap, and the winding is connected to two of the corner taps of a rectifier bridge, while the charging and discharging circuits of the condenser are connected to the other two corner taps of the bridge, the rectifier bridge causing both the charging and the discharging impulses to pass through the winding in the same direction.

By synchronizing the change-over switch or switches with the A.C. voltage so that the condenser is charged and discharged 4 times in each A.C. voltage cycle and by arranging the periods of charging impulses in pairs symmetrically about the respective maximum and the minimum of the successive half-cycles of the sine curve, all the impulses fed to the X-ray tube will be of substantially the same magnitude.

In still another embodiment of the apparatus, a parallel combination of a resistance and an inductance is connected in series with the condenser such that the active primary voltage may, by providing proper values or sizes of the components, be maintained substantially constant during the length of the impulses.

In still another embodiment of the invention a choke coil with a relatively sharp-edged magnetization curve is connected in parallel with the primary winding or windings of the transformer, and this choke coil may be provided with a value to be saturated at the end of the desired impulse, thus cutting off that part of the impulse which heats the X-ray tube without producing usable X-rays. A similar effect is achieved by an embodiment in which a resistor in series with an electronic switch is connected in parallel to the primary winding or windings, said switch being coupled to and controlled either directly or indirectly by the electron flow in the X-ray tube. By controlling the electronic switch by the voltage over a parallel combination of a resistor and a condenser inserted in series with the X-ray tube and heating the cathode of the tube by impulses proportional to the anode-voltage impulses, an automatic adjustment of the temperature of the cathode and hence of the anode current of the X-ray tube is achieved.

As already mentioned, the condenser forms, together with the active primary winding, a resonant circuit in which attenuated oscillations are produced at each impulse. These oscillations have the useful function of demagnetizing the transformer core after each impulse. If this were not done, each impulse would leave the core in a more or less magnetized state, and all impulses magnetizing in the same direction, the next impulse would then produce a relatively small change of flux and hence a relatively small secondary voltage.

This invention will be explained in greater detail with reference to the appended drawing, in which,

FIGURE 1 illustrates a known spark coil circuit for feeding an X-ray tube;

FIGURE 2 shows the impulses produced by such a circuit;

FIGURE 3 is an embodiment of the X-ray apparatus according to the invention adapted for connection to a DC. supply;

FIGURE 4 shows an impulse produced by the apparatus of FIG. 3;

FIGURE 5 is a modification of the apparatus shown in FIG. 3;

FIGURE 6 shows an impulse produced by the apparatus shown in FIG. 5;

FIGURE 7 represents a sinusoidal A.C. voltage curve;

FIGURE 8 is a further embodiment of the apparatus according to the invention adapted for connection to an A.C. supply;

FIGURE 9 shows charging and discharging curves for the condenser of the apparatus of FIG. 8;

FIGURE 10 shows a modification of the apparatus shown in FIG. 8; and

FIGURE 11 is a further embodiment of the apparatus according to the invention adapted for connection to an A.C. supply.

In FIG. 1 a motor 1 drives a change-over switch 2 by means of which a condenser 3 is alternately charged from a DC. source and discharged. Both the charging and the discharging current pass through the primary winding of a spark coil 7, but in opposite directions, whereby the voltage impulses illustrated in FIG. 2 are produced across the secondary winding.

In the embodiment of the apparatus according to the invention illustrated in FIG. 3 the high-voltage transformer 7a has two primary windings 5 and 6 which are connected ot the DC. voltage source and to a changeover switch 2a such that the charging current of the condenser 3a will pass through one primary winding and the discharging current through the other, and so that the two currents will produce fluxes in the same direction in the transformer core and hence secondary voltage impulses of the same polarity. Such an impulse is illus trated in FIG. 4.

In FIG. 3 there is shown a parallel combination of a resistor 8 and a choke coil 9 connected in series with the condenser 3a. Without this parallel combination of the secondary impulses would have the form indicated by the dotted line in FIG. 4. The resistor 8, however, produces at each contact-making of the switch, a voltage drop, the magnitude of which decreases gradually as the choke coil 9 takes over the current. By a suitable adjustment of the value of the components and particularly by utilizing saturation of the core of the choke coil, the parallel combination may be caused to compensate the condenser voltage variation for a certain period, whereby the produced impulses will have the form shown by the full lines in FIG. 4 and corresponding to a practically constant high-voltage during the period of impulse duration.

The part of the first positive impulse in FIG. 4 following the moment t causes heating of the X-ray tube (not shown) without producing usable X-rays. This useless heating may be reduced by the embodiment illustrated in FIG. 5, which is distinguished from the embodiment of FIG. 3 in that the primary windings 5b and 6b are not located directly on the high-voltage transformer, but on a separate transformer 12, the secondary winding 10 of which produces a relatively low impulse voltage and is connected to the primary winding of the high-voltage transformer 7b. Connected in parallel to the secondary winding 10 is moreover an adjustable choke coil 11 having a Sharpe-edged magnetization curve, as for example, provided with a nickel-iron core. This core is saturated at the moment t whereby the impedance of the choke coil will drop significantly and terminate the impulse, as shown in FIG. 6.

FIG. 8, shows an embodiment of the X-ray apparatus according to the invention adapted for connection with an A.C. supply. Therein, the X-ray tube is designated by numeral 21 and the high-voltage transformer by numeral 14. The primary winding of the transformer 14 is connected to the two corner outlet taps of a rectifier bridge 13, the other corner taps of which are connected respectively to the condenser 3c and the movable arm of the changeover switch 20. In FIG. 7 a cycle of the AC. supply voltage is shown divided into eight intervals of equal length, and the points of division are consecutively numbered. The change-over switch 2c is driven in synchronization with the A.C. voltage so that at all uneven times its arm will make contact with the upper contact member 15 and at all even times with the lower contact member 16, whereby the condenser 30 will be alternately charged and discharged. By means of the rectifier bridge 13, all charging and discharging currents are made to flow in the same'direction through the primary winding of the transformer, no matter whether the condenser voltage has one or the other polarity. It will be seen that the condenser is charged and discharged four times in each period of the A.C. voltage corresponding to the generation of eight high-voltage impulses of the same polarity for feeding the X-ray tube 21. It should be noted that the uneven points in FIG. 7 are arranged in pairs symmetrically about the maximum and the minimum values of the sine curve respectively, to produce impulses of equal magnitude.

The condenser voltage as a function of time is illustrated in FIG. 9. The generated anode voltage is proportional to the rate of change of the condenser voltage. Since anode voltagesbelow a certain limit AU, as previously mentioned, produce heat, but no usable rays in the X-ray tube, it will be expedient to discontinue the impulses except at the time t as is done in the embodiment shown in FIG. 5. This may be effected by means of the additional circuits illustrated in FIG. 10 and comprising a thyratron tube 18, which in series with a resistor 19 is connected in parallel to the primary winding of the transformer 14d, and the ignition movement of which is controlled by the voltage drop over a parallel combination of a resistor 22 and a condenser 23 inserted in series with the X-ray tube 21d and forming a timeconstant circuit. The current impulse in the X-ray tube circuit produces a gradually increasing voltage drop across the RC combination of resistor 22 and condenser 23, and when the voltage drop reaches a predetermined value, the thyratron tube 18 is ignited, and the main part of the condenser current will then pass through the resistor 18 instead of through the primary winding of the transformer.

In FIG. 8 and FIG. 10 the cathode of the X-ray tube is fed from a tap on the secondary winding of the highvoltage transformer. If the anode current of the X-ray tube in FIG. 10 gets too weak, the time-constant circuit constituted of resistor 22 and condenser 23 will cause the impulse to be prolonged, whereby the temperature of the cathode and consequently the anode current will rise. In this manner the operation of the X-ray tube is adjusted automatically without the use of a special adjustable heating transformer.

A by-pass condenser 20 and a series choke coil 17 will prevent the charging and discharging impulses of the condenser 3d from propagating to the feed voltage source to any disturbing extent. The choke coil 17 may be given a value to produce, jointly with the condenser 20, resonance effects which increase the feed voltage.

FIG. 11 illustrates still another embodiment of the apparatus according to the invention which is distinguished from the embodiment shown in FIG. 5 only in that it is adapted to be operated by A.C. For this purpose the primary windings 5e and 6c of the first transformer are provided with center taps 24- and 25 connected to the terminals of the AC. voltage source, and the changeover switch 211 in FIG. 5 has been replaced by two changeover switches 26 and 27, the movable arms of which are jointly connected to the condenser 3e, while the fixed contact members 28-31 are connected to the ends of the primary windings 5e and 6e as shown. The change-over switches are operated in synchronism with the AC. voltage so that the arm 27 makes contact with 30 at points 1 and 3 in FIG. 7 and with 31 at points 2 and 4, whereas the arm 26 makes contact with 28 at points 5 and 7 and with 2? at points 6 and 8. Thereby all the charging and discharging currents of the condenser 32, irrespective of their direction in relation to the condenser, will produce fluxes in the same direction in the core of the first transformer in the same manner as in the embodiments shown in FIG. 8 and FIG. 10. The components 8e, 9e, and 11:; function in the same manner as components 8, 9, and 11 and serve the same purposes as explained above with reference to FIG. 5.

The invention is not limited to the embodiments which are described above an illustrated in the drawings, and

many modifications of these embodiments are possible within the scope of the invention. The changeover switches 2, 26 and 27, for example, may be electronic switches instead of being mechanical and motor-driven.

The invention covers all modifications and variations which fall within the spirit and scope of the invention as defined by the appended claims.

What is claimed is:

1. X-ray apparatus comprising a high-voltage transformer and an X-ray tube which is connected directly to the high-voltage transformer, said transformer having at least one primary winding, an AC. voltage source, a circuit connecting the AC. voltage source and said primary winding; a single pole double throw switch in said circuit, a condenser in said circuit, said switch having a first position in which the condenser is connected to the AC. source and is charged thereby and a second position in which the condenser is isolated from the source and undergoes discharge, and means coupled to the condenser and the transformer winding to produce fluxes in the same direction in the primary winding of the transformer during both charging and discharging of the condenser, said switch being driven in synchronized relation with the frequency of the AC. voltage such that the condenser is charged and discharged 4 times in each AC. voltage cycle, the charging periods being arranged in respective symmetrical pairs about the maximum and the minimum of the voltage sine curve.

2. X-ray apparatus according to claim 1 wherein said means for producing fluxes in the same direction in the primary winding of the transformer comprises a rectifier bridge having pairs of input and output terminals, the primary Winding of the transformer being connected to one of the pairs of terminals of the rectifier bridge, while said circuit containing the condenser is connected to the other of the pairs of terminals of the bridge.

3. X-ray apparatus according to claim 1 comprising a resistor in series with electronic switch means connected in parallel with the primary winding, said switch means being coupled with and controlled by the electron flow in the X-ray tube.

4. X-ray apparatus according to claim 3 comprising a parallel combination of a resistor and a condenser inserted in series with the X-ray tube controlling the electronic switch means, the X-ray tube having a cathode which is heated by impulses proportional to the anode voltage impulses.

5. An electrical power supply for an X-ray tube comprising a transformer having a secondary winding adapted for connection with an X-ray tube and two primary windings, an AC. voltage source having terminals connected to respective center taps of the primary windings, first and second single pole double throw switches, each switch having two fixed contacts one of which is connected to a respective end of one of the primary windings while the other is connected to a respective end of the other primary winding, said switches having movable arms, a condenser connected to one terminal of the AC. voltage source and to the movable arms of the switches, said arms being alternately operative in synchronization with the frequency of the AC. voltage such that said arms alternately contact the associated fixed contacts twice during respective half-cycles of the A.C. voltage and the condenser is charged and discharged four times in each AC. voltage cycle and flux is produced in the same direction in the primary windings of the transformer during charging and discharging of the condenser, the charging periods being arranged in respective symmetrical pairs about the maximum and the minimum of the voltage sine curve.

6. Apparatus according to claim 5 comprising a parallel combination of a resistance and an inductance connected in series between the condenser and the AC. voltage source.

7. Apparatus according to claim 5 comprising a choke coil having a relatively sharp-edged magnetization characteristic connected in parallel with the secondary winding of the transformer.

8. A method of supplying an X-ray tube with voltage impulses, said method comprising: supplying the X-ray tube with impulses from the secondary winding of a transformer, and feeding the primary winding of the transformer with successive pulses which produce fluxes in the same direction in the primary winding, the pulses delivered to the primary winding being produced in synchronization with the frequency of an AC. supply source, the pulses being delivered at least one for each half-cycle during only a portion of each half-cycle and with substantially uniform voltage magnitude.

9. A method as claimed in claim 8 wherein a plurality of pulses are delivered during each half-cycle, the pulses being arranged in time-wise symmetry with the voltage maximum and minimum in the respective successive halfcycles.

References Cited by the Examiner UNITED STATES PATENTS 2,146,977 2/ 1939 Nilsson 315-223 2,184,315 12/1939 Peters et a1. 315223 2, 823,319 2/1958 Vossberg 25098 RALPH G. NILSON, Primary Examiner. A. L. BIRCH, Assistant Examiner. 

1. X-RAY APPARATUS COMPRISING A HIGH-VOLTAGE TRANSFORMER AND AN X-RAY TUBE WHICH IS CONNECTED DIRECTLY TO THE HIGH-VOLTAGE TRANSFORMER, SAID TRANSFORMER HAVING AT LEAST ONE PRIMARY WINDING, AN A.C. VOLTAGE SOURCE, A CIRCUIT CONNECTING THE A.C. VOLTAGE SOURCE AND SAID PRIMARY WINDING; A SINGLE POLE DOUBLE THROW SWITCH IN SAID CIRCUIT, A CONDENSER IN SAID CIRCUIT, SAID SWITCH HAVING A FIRST POSITION IN WHICH THE CONDENSER IS CONNECTED TO THE A.C. SOURCE AND IS CHARGED THEREBY AND A SECOND POSITION IN WHICH THE CONDENSER IS ISOLATED FROM THE SOURCE AND UNDERGOES DISCHARGE, AND MEANS COUPLED TO THE CONDENSER AND THE TRANSFORMER WINDING TO PRODUCE FLUXES IN THE SAME DIRECTION IN THE PRIMARY WINDING OF THE TRANSFORMER DURING BOTH CHARGING AND DISCHARGING OF THE CONDENSER, SAID SWITCH BEING DRIVEN IN SYNCHRONIZED RELATION WITH THE FREQUENCY OF THE A.C. VOLTAGE SUCH TT THE CONDENSER IS CHARGED AND DISCHARGED 4 TIMES IN EACH A.C. VOLTAGE CYCLE, THE CHARGING PERIODS BEING ARRANGED IN RESPECTIVE SYMMETRICAL PAIRS ABOUT THE MAXIMUM AND THE MINIMUM OF THE VOLTAGE SINE CURVE. 